1. Field of the Invention
This invention relates generally to a true time delay (TTD) line and, more particularly, to a TTD line circuit including one or more Archimedean spiral delay lines and components for providing electric and/or magnetic isolation between the delay lines.
2. Discussion of the Related Art
TTD lines are electrical devices that delay an electrical signal, such as an RF signal, for a defined period of time. Standard TTD technology employs digitally switched transmission line sections where weight, loss and cost increase rapidly with increased operational frequency and/or phase tuning resolution.
TTD lines have application for many electrical circuits and systems, especially wideband systems. For example, TTD lines have application for wideband pulse electronic systems, where the TTD line provides an invariance of a time delay with frequency or a linear phase progression with frequency. In this application, the TTD line allows for a wide instantaneous signal bandwidth with virtually no signal distortion, such as pulse broadening during pulsed operation.
TTD lines also have application in wideband phased array antenna systems. These types of phased arrays provide beam steering where the direction of the antenna beam can be changed or scanned for the desired application. As the beam radiation pattern changes, the phase of the received signals at the node from different antenna elements also changes, which needs to be corrected. Phase shifters can be provided for each antenna element for this purpose. The frequency and bandwidth of a conventional phased antenna array is altered or limited by the bandwidth of the array elements, where limitations are caused by the use of the phase shifters to scan the antenna beam. TTD lines can be employed in the place of phase shifters to provide a delay in the transmitted and received signals to control the phase. The use of TTD lines potentially eliminates the bandwidth restriction by providing a theoretically frequency independent time delay on each antenna element channel of the array.
The most distinct advantage of a TTD based phased array is the elimination of the beam squint effect. Compared to those phase shifter based phased arrays, TTD based phased arrays can simultaneously operate at various frequencies over a very wide bandwidth without losing precision of antenna directionality across the entire band.
There are a number of techniques and designs in the art for providing TTD lines. For example, high temperature superconductor delay line structures have been disclosed. One particular structure of this type includes two substrates having thin film strips on opposing sides that are in contact with each other to implement a single strip-line circuit, which provides an air gap between the substrates. However, this type of design provides a narrow RF line width that increases overall signal loss. If a wider strip line is used, then extra long tapered transformer sections are required to interface with 50 ohm systems, which causes extra size and loss that complicate the design. Further, there are related manufacturing issues in that only periodic contacts exist on the RF traces. Also, accumulative cross-talk and forward/backward coupling may be a problem. The design is also typically expensive to deploy and difficult to integrate with other components and systems.
Coaxial delay lines are also known in the art and have long been used in electronic systems to delay, filter or calibrate signals. Coaxial delay lines can be provided in many different sizes and formed into countless configurations. Certain front-end designs can improve cost, size, configuration and overall electrical performance of not just the delay line, but the overall system. However, coaxial delay lines are typically not suitable for planar integration, are difficult to mechanically form and have a velocity factor that is higher than most commercially available coaxial cables.
Other known TTD lines include delay lines having a constant resistance, varactor non-linear transmission line (NLTL) tunable delay lines, ferro-electric substrate tunable delay lines, dielectric filled waveguide delay lines, surface acoustic wave (SAW) delay lines, air line inside a PCB three-dimensional coaxial structure delay line, micro-electro-mechanical system (MEMS) tunable transmission delay lines, meta material structure synthesized transmission delay lines, photonics delay lines, resonator structure delay lines, and digital time delay lines.
However, each of these TTD line designs suffers one or more drawbacks that make it at least somewhat undesirable for wideband applications, such as wideband phased array antenna systems. For example, delay lines having a constant resistance are typically limited to lower microwave frequency bands and are very lossy. Varactor NLTL tunable delay lines have issues with the varactors, a small time delay range, and are difficult to tune because of being continuous in a digital command world. Ferro-electric substrate tunable delay lines have problems with linearity, require very high voltages, have variable impediments and return losses, and are difficult for providing as much delay as desired. Dielectric filled waveguide delay lines are typically very heavy and bulky for practical applications. SAW delay lines are typically difficult to implement at high frequencies, provide too much signal loss and are difficult to manufacture. Air line coaxial structure delay lines are typically heavy and bulky to be practical. MEMs tunable transmission lines typically have too small of a delay time, are often unreliable and require high voltages. Meta material structure synthesized transmission lines typically are very narrow band. Photonics delay lines typically require too much power and have significant RF losses. Resonant structure delay lines are typically difficult to provide both wide bandwidth and high delay at the same time. Digital time delay lines typically have high power consumption.
What is needed is a TTD line that provides all of the desired qualities for wideband applications, such as significant delay, ease of manufacture for monolithic integration, ease for multi-bit delay implementation, low weight, low cross-talk, forward/backward coupling, low radiation level, small size, ultra-wide bandwidth, low losses, low cost, etc.